1. Field of the Invention
This invention relates generally to a membrane for hydrogen recovery from streams containing hydrogen sulfide and, more particularly, the invention relates to a membrane for hydrogen recovery from streams containing hydrogen sulfide which contain transition metal sulfides having resistance to sulfur compounds such as H2S, the ability to catalyze the decomposition of H2S at moderate temperatures, the ability to catalyze the photochemical decomposition of water, and thus H2S, to hydrogen at room temperatures, and the ability to adsorb hydrogen.
2. Description of the Prior Art
Most industrial processes for the recovery of sulfur from streams containing hydrogen sulfide are based on the Claus process. This entails partial combustion of H2S, which is stripped, for example, from a sour natural gas stream, or from the effluent of a hydrodesulfurization unit in an oil refinery, to form SO2. Elemental sulfur is recovered by reacting the remaining H2S with SO2. Thermodynamic constraints limit the conversion (to about 0.7) of H2S and, hence, the thermal recovery of elemental sulfur from the Claus furnace (operated at around 2400° F.). The effluent gases from the Claus furnace are cooled to recover sulfur and then contacted over a catalyst bed at lower temperatures to increase the efficiency of sulfur recovery. For optimum operation, the composition of the gases in the Claus process must be maintained such that the H2S/SO2 ratio is 2:1. Even after several conversion stages, 2000–3000 ppm of H2S and SO2 may remain in the effluent gas from the Claus process. An additional tail-gas cleanup unit (TGCU), which could cost roughly as much, and up to three times more than the Claus unit, depending on the size of the plant and sulfur recovery required—has to be used to ensure that the final overall sulfur recovery exceeds ninety-nine (99%) percent.
A major disadvantage, besides limited sulfur recovery, of the Claus process is that the energy contained in the hydrogen sulfide is lost. The economics of handling many sour-gas wells and H2S-containing streams could be improved if the hydrogen sulfide could be effectively split into hydrogen and sulfur. The hydrogen would then be:                recycled to the refinery for use in hydrogenation applications; and        used as a clean fuel in a fuel cell, or in direct combustion applications.However, the direct thermal decomposition of H2S to produce hydrogen and sulfur also suffers from limitations imposed by slow reaction rates, even at high temperatures. Thus, the conversion is only a few percent at 800° C., and temperatures in excess of 2000° C. are required for complete thermolysis. The corrosive (and toxic) nature of H2S imposes additional limitations on high-temperature materials that may be used.        
An alternative is to use catalysts that enhance the H2S-decomposition rates. For example, sulfided transition metals and their mixtures have been found to be effective. More than ninety-five (95%) percent conversion of H2S was reported with molybdenum disulfide (MoS2) at 800° C. with continuous removal of sulfur and intermittent removal of hydrogen. The role of the sulfides of chromium, cobalt, nickel, and iron has also been investigated; chromium sulfide exhibited stabilized activity. In all these cases, the total residence time in the reactor required to achieve the high conversions is of the order of hours. This, of course, limits large-scale practical application.
Another option is the use of membrane (catalytic, or non-catalytic) reactors. Many applications in the petroleum refineries have the potential to benefit from use of membranes that provide selective permeation of hydrogen. Besides providing a readily used, relatively pure hydrogen stream, this configuration also permits overcoming of (closed system) equilibrium conversion limitations. A severe constraint in terms of the application to H2S-containing streams is that membrane materials; for example, palladium and/or its alloys that have been found effective for hydrogen permeation lose their catalytic activity, capability for hydrogen sorption, permeability, and structural integrity.
The use of membrane reactors for H2S decomposition is known. Relatively small enhancements in equilibrium conversion, from six (6%) percent to twelve (12%) percent at 800° C., have been found using a ceramic (quartz) membrane reactor walls. Close to one hundred (100%) conversion of 1.5% H2S at 115 psia using dense (metallic) membrane reactor walls has also been found. Key to achieving this result was a H2-permeable composite membrane formed by deposition of a sulfur-resistant platinum coating on hydrogen-permeable vanadium on the feed side. Others have confirmed that a platinum-coated membrane is structurally satisfactory at high temperatures of interest, its permeability drops considerably after only a short period of operation. Therefore, unfortunately, the use of palladium-coated membranes in these studies led to rupture, and material failure.